This all depends on what you mean by 'proper wpg'. In its purest sense you would just take the total number of watts over the aquarium divided by the number of gallons (20). If you are wanting to try to figure out how that number relates to the 'wpg rule', then you would need to use an appropriate conversion from PC to T12. If you don't have the necessary information on the bulb to be able to compare efficiency, then 1.41 is a reasonable multiplier based on a sampling of PC bulbs.
A 20 gallon long is large enough that those calculations will give you a descent estimate of the amount of light over your aquarium. There are multiple other factors that come into play, like the quality of the reflector, distance of the bulbs from the surface of the water, and whether or not you're using a glass canopy. These and many other factors will play a large part in the functional light level in your aquarium.